Prevalence and activation phenotype of Sendai virus-specific CD4+ T cells.

The relative prevalence of Sendai virus-specific CD4+ T helper precursors (Thp) has been assessed by limiting dilution analysis (LDA). Within 10 days after intranasal inoculation of C57BL/6 mice, Thp prevalence increased from < 1 in 10(5) to approximately 1 in 200 in CD4+ T-cell-enriched spleen and mediastinal lymph node populations. These frequencies remained elevated relative to naive animals, with the majority of the Thp recovered at 2-3 months after infection being found in the spleen. The "memory" Thp express an "activated" L-selectin-low, CD44-high, alpha 4-integrin-high phenotype, comparable to that described previously for CD8+ cytotoxic T lymphocyte precursors (CTLp) specific for Sendai virus. Background effects for the IL-2-based Thp analysis are, however, less predictable than those found previously for the 51Cr release CTLp LDA, as the extent of non-virus-specific lymphokine production varies. However, provided the analysis is appropriately controlled, the approach does allow useful comparisons between phenotypically different subsets of antigen-specific CD4+ T cells.